CD70 is a member of the tumor necrosis factor (TNF) family of cell membrane-bound and secreted molecules that are expressed by a variety of normal and malignant cell types. The primary amino acid (AA) sequence of CD70 predicts a transmembrane type II protein with its carboxyl terminus exposed to the outside of cells and its amino terminus found in the cytosolic side of the plasma membrane (Bowman et al., 1994, J. Immunol. 152:1756-61; Goodwin et al., 1993, Cell 73:447-56). Human CD70 is composed of a 20 AA cytoplasmic domain, an 18 AA transmembrane domain, and a 155 AA extracytoplasmic domain with two potential N-linked glycosylation sites (Bowman et al., supra; Goodwin et al., supra). Specific immunoprecipitation of radioisotope-labeled CD70-expressing cells by anti-CD70 antibodies yields polypeptides of 29 and 50 kDa (Goodwin et al., supra; Hintzen et al., 1994, J. Immunol. 152:1762-73). Based on its homology to TNF-alpha and TNF-beta, especially in structural strands C, D, H and I, a trimeric structure is predicted for CD70 (Petsch et al., 1995, Mol. Immunol. 32:761-72).
Original immunohistological studies revealed that CD70 is expressed on germinal center B cells and rare T cells in tonsils, skin, and gut (Hintzen et al., 1994, Int. Immunol. 6:477-80). Subsequently, CD70 was reported to be expressed on the cell surface of recently antigen-activated T and B lymphocytes, and its expression wanes after the removal of antigenic stimulation (Lens et al., 1996, Eur. J. Immunol. 26:2964-71; Lens et al., 1997, Immunology 90:38-45). Within the lymphoid system, activated natural killer cells (Orengo et al., 1997, Clin. Exp. Immunol. 107:608-13) and mouse mature peripheral dendritic cells (Akiba et al., 2000, J. Exp. Med. 191:375-80) also express CD70. In non-lymphoid lineages, CD70 has been detected on thymic medullar epithelial cells (Hintzen et al., 1994, supra; Hishima et al., 2000, Am. J. Surg. Pathol. 24:742-46).
In addition to expression on normal cells, CD70 expression has been reported in different types of cancers including lymphomas, carcinomas, and tumors of neural origin. In malignant B cells, 71% of diffuse large B-cell lymphomas, 33% of follicle center lymphomas, 25% of mantle lymphomas, and 50% of B-CLL have been reported to express CD70 (Lens et al., 1999, Br. J. Haematol. 106:491-503). CD70 is frequently expressed together with other lymphoid activation markers on the malignant Hodgkin and Reed-Sternberg cells of Hodgkin's disease (Gruss and Kadin, 1996, Bailieres Clin. Haematol. 9:417-46). One report demonstrates CD70 expression on 88% (7 of 8 cases) of thymic carcinomas and 20% (1 of 5 cases) of atypical thymomas (Hishima et al., 2000, supra). The second type of carcinoma on which CD70 has been detected is nasopharyngeal carcinoma. One study reports the presence of CD70 on 80% (16 of 20 cases) of snap-frozen tumor biopsies obtained from undifferentiated nasopharyngeal carcinomas (Agathanggelou et al., 1995, Am J Path 147:1152-60). CD70 has also been detected on brain tumor cells, especially glioma cell lines, solid human gliomas, and meningiomas (Held-Feindt and Mentlein, 2002, Int. J. Cancer 98:352-56; Wischlusen et al., 2002, Can. Res. 62:2592-99).
The receptor for CD70 is CD27, a glycosylated type I transmembrane protein of about 55 kDa (Goodwin et al., 1993, Cell 73:447-56; Hintzen et al., 1994, supra). CD70 is sometimes referred to as CD27L. CD27, which exists as a homodimer on the cell surface (Gravestein et al., 1993, Eur. J. Immunol. 23:943-50), is a member of the TNF receptor superfamily as defined by cysteine-rich repeats of about 40 amino acids in the extracellular domain (Smith et al., 1990, Science 248:1019-23; Locksley et al., 2001, Cell 104:487-501). CD27 is expressed by thymocytes, NK, T, and B cells (Hintzen et al., 1994, Immunol. Today 15:307-11; Lens et al., 1998, Semin. Immunol. 10:491-99). On resting T cells, CD27 is constitutively expressed, yet antigenic triggering further upregulates CD27 expression (de Jong et al., 1991, J. Immunol. 146:2488-94; Hintzen et al., 1993, J. Immunol. 151:2426-35). Further, triggering of T cells via their T cell antigen receptor complex alone or in combination with the accessory molecule CD28 releases soluble CD27 from activated T cells (Hintzen et al., 1991, J. Immunol. 147:29-35). Naïve B cells do not express CD27, but its expression is induced and, in contrast to CD70, sustained after antigenic triggering of B cells (Jacquot et al., 1997, J. Immunol. 159:2652-57; Kobata et al., 1995, Proc. Natl. Acad. Sci. USA 92:11249-53).
In marked contrast to the restricted expression of CD27 and CD70 in normal B lineage cells, both CD27 and CD70 are frequently co-expressed in many B cell non-Hodgkin's lymphomas and leukemias. This could potentially lead to functional CD27-CD70 interactions on these cells in the form of an autocrine loop, resulting in CD27 signaling and in CD70-induced proliferation, thereby providing a growth advantage to malignant cells (Lens et al., 1999, supra).
The role of CD70-CD27 co-stimulation in cell-mediated autoimmune diseases has been investigated in a model of experimental autoimmune encephalomyelitis (EAE) (Nakajima et al., 2000, J. Neuroimmunol. 109:188-96). In vivo administration of the anti-mouse CD70 mAb (clone FR-70) suppressed the onset of EAE by inhibiting antigen-induced TNF-alpha production without affecting B and T cell number, T cell priming, Ig production or TH1/TH2 cell balance. However, such treatment had little efficacy in established disease.
Graft versus host disease (GVHD) is a TH1-mediated immune response that is a major and often lethal consequence of allogeneic bone marrow transplantation (BMT) therapy that occurs when histocompatibility antigen differences between the BM donor and the recipient of the transplant are present (den Haan et al., 1995, Science 268:1476). GVHD is an immune reaction against host tissues mounted by mature T cells present in the transplanted donor marrow (Giralt and Champlin, 1994, Blood 84:3603). It is noteworthy that CD70 has been detected in vivo on CD4+ cells in conditions characterized by allogeneic reaction, as in cases of maternal T cell engraftment in severe combined immune deficiency patients (Brugnoni et al., 1997, Immunol. Lett. 55:99-104). Prophylaxis of GVHD is achieved by pan-T cell immunomodulatory agents such as cyclosporine, corticosteroids, or methotrexate. However, these agents are not specific and cause significant adverse side effects.
As indicated supra, CD70 is not expressed on normal non-hematopoietic cells. CD70 expression is mostly restricted to recently antigen-activated T and B cells under physiological conditions, and its expression is down-regulated when antigenic stimulation ceases. Evidence from animal models suggests that CD70 may contribute to immunological disorders such as, e.g., rheumatoid arthritis (Brugnoni et al., 1997, Immunol. Lett. 55:99-104), psoriatic arthritis (Brugnoni et al., 1997, Immunol. Lett. 55:99-104), and systemic lupus erythematosus (SLE). (Oelke et al., 2004, Arthritis Rheum. 50:1850-60). In addition to its potential role in inflammatory responses, CD70 is also expressed on a variety of transformed cells including lymphoma B cells, Hodgkin and Reed-Sternberg cells, malignant cells of neural origin, and a number of carcinomas.
Accordingly, there is a need for anti-CD70 antibodies and other CD70 binding agents that can exert a clinically useful cytotoxic, cytostatic, or immunomodulatory effect on CD70-expressing cells, particularly without exerting undesirable effects on non-CD70-expressing cells. Such binding agents would be useful against cancers that express CD70 or immune disorders that are mediated by CD70-expressing cells.